</?a ?([^>]+)?>
The question marks are used to mark what comes before as optional.
So </?a is saying that the slash before the 'a' tag is optional. That means it matches both "<a"and "</a".
Then comes the space, which is also made optional, meaning it will match "<a", or "<a ".
The ([^>]+)? is a little more tricky, but not terribly so. The parentheses are used to group everything before the last question mark. Meaning the whole of what's inside the parentheses is optional.
"[^>]" is a common character class when trying to parse html tags. It simply means that it will match any character that is not (^) the greater-than character (>). It's used to ensure that the expression does not get greedy and grab content beyond the ending of the current tag (>). The + is for repetition. + is one or more times, and * means 0 or more times.
The use of + in this case is why the grouping parentheses and the question mark to make the whole thing optional is necessary. In this particular case: the optional space character and the ([^>]+)? could be replaced with simply [^>]*
(meaning match all characters (except >) zero or more times, instead of all characters (except >) one or more times...
optionally).
Then match the closing > character.
</?a ?([^>]+)?>
should be synonymous with:
</?a[^>]*>
for the stripping of all opening and closing anchor tags (as well as any self-closing anchor tags of the variety: <a id="anchor_tag_1" />)
But no need to change what works. I included the slight simplification for explanatory purposes.
